This invention relates to the clamping circuitry of a servomotor i.e., to a circuit configuration for limiting the cutoff voltage occurring in controlling a servomotor by means of width-modulated DC pulses.
As is known, overvoltages, caused by the discharge of the energy stored in an inductive load, appear upon cutoff of inductive loads or of partially inductive loads, including electric motors. In DC networks, it is possible to limit the overvoltage by means of a free-wheel diode connected in parallel with the load and acted upon by the DC voltage in the nonconducting direction. However, this measure does not work for a DC motor which also is to be operated as a generator or to be reset unrestrictedly by a mechanical force.
The present invention relates to an electric servomotor for the throttle valve of an automotive vehicle engine. Such a servomotor, for exampled is used for controlling or regulating the drive performance of an engine in conjunction with a traction slip control system. The servomotor is preferably switched on and off via a power transistor, control being effected by width-modulated current pulses. A clamping circuit is needed for the servomotor in order to protect the final stage transistor against overvoltages which are generated when the motor is turned off or when the final stage transistor becomes non-conductive during width-modulated pulse control.
However, cutoff voltage limitation by means of a free-wheel diode cannot be used when the servomotor acts against a tension spring and is to be reset by the spring force into the zero position as quickly as possible. Short-circuiting of the winding of the servomotor via such a free-wheel diode would counteract the return motion of the motor and consequently obstruct a quick resetting of the throttle valve by means of the spring force or a quick position control of the throttle valve.